PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)NAMEperldebug - Perl debugging
DESCRIPTION
First of all, have you tried using the -w switch?
The Perl Debugger
If you invoke Perl with the -d switch, your script runs
under the Perl source debugger. This works like an inter
active Perl environment, prompting for debugger commands
that let you examine source code, set breakpoints, get
stack backtraces, change the values of variables, etc.
This is so convenient that you often fire up the debugger
all by itself just to test out Perl constructs interac
tively to see what they do. For example:
$ perl -d -e 42
In Perl, the debugger is not a separate program the way it
usually is in the typical compiled environment. Instead,
the -d flag tells the compiler to insert source informa
tion into the parse trees it's about to hand off to the
interpreter. That means your code must first compile cor
rectly for the debugger to work on it. Then when the
interpreter starts up, it preloads a special Perl library
file containing the debugger.
The program will halt right before the first run-time exe
cutable statement (but see below regarding compile-time
statements) and ask you to enter a debugger command. Con
trary to popular expectations, whenever the debugger halts
and shows you a line of code, it always displays the line
it's about to execute, rather than the one it has just
executed.
Any command not recognized by the debugger is directly
executed ("eval"'d) as Perl code in the current package.
(The debugger uses the DB package for keeping its own
state information.)
For any text entered at the debugger prompt, leading and
trailing whitespace is first stripped before further pro
cessing. If a debugger command coincides with some func
tion in your own program, merely precede the function with
something that doesn't look like a debugger command, such
as a leading ";" or perhaps a "+", or by wrapping it with
parentheses or braces.
Debugger Commands
The debugger understands the following commands:
h [command] Prints out a help message.
If you supply another debugger command as an
argument to the "h" command, it prints out the
description for just that command. The spe
cial argument of "h h" produces a more compact
help listing, designed to fit together on one
screen.
If the output of the "h" command (or any
command, for that matter) scrolls past your
screen, precede the command with a leading
pipe symbol so that it's run through your
pager, as in
DB> |h
You may change the pager which is used via "O
pager=..." command.
p expr Same as "print {$DB::OUT} expr" in the current
package. In particular, because this is just
Perl's own "print" function, this means that
nested data structures and objects are not
dumped, unlike with the "x" command.
The "DB::OUT" filehandle is opened to
/dev/tty, regardless of where STDOUT may be
redirected to.
x expr Evaluates its expression in list context and
dumps out the result in a pretty-printed fash
ion. Nested data structures are printed out
recursively, unlike the real "print" function
in Perl. See the Dumpvalue manpage if you'd
like to do this yourself.
The output format is governed by multiple
options described under the section on "Con
figurable Options".
V [pkg [vars]]
Display all (or some) variables in package
(defaulting to "main") using a data pretty-
printer (hashes show their keys and values so
you see what's what, control characters are
made printable, etc.). Make sure you don't
put the type specifier (like "$") there, just
the symbol names, like this:
V DB filename line
Use "~pattern" and "!pattern" for positive and
negative regexes.
This is similar to calling the "x" command on
each applicable var.
X [vars] Same as "V currentpackage [vars]".
T Produce a stack backtrace. See below for
details on its output.
s [expr] Single step. Executes until the beginning of
another statement, descending into subroutine
calls. If an expression is supplied that
includes function calls, it too will be sin
gle-stepped.
n [expr] Next. Executes over subroutine calls, until
the beginning of the next statement. If an
expression is supplied that includes function
calls, those functions will be executed with
stops before each statement.
r Continue until the return from the current
subroutine. Dump the return value if the
"PrintRet" option is set (default).
<CR> Repeat last "n" or "s" command.
c [line|sub]
Continue, optionally inserting a one-time-only
breakpoint at the specified line or subrou
tine.
l List next window of lines.
l min+incr List "incr+1" lines starting at "min".
l min-max List lines "min" through "max". "l -" is syn
onymous to "-".
l line List a single line.
l subname List first window of lines from subroutine.
subname may be a variable that contains a code
reference.
- List previous window of lines.
w [line] List window (a few lines) around the current
line.
. Return the internal debugger pointer to the
line last executed, and print out that line.
f filename Switch to viewing a different file or "eval"
statement. If filename is not a full pathname
found in the values of %INC, it is considered
a regex.
"eval"ed strings (when accessible) are consid
ered to be filenames: "f (eval 7)" and "f eval
7\b" access the body of the 7th "eval"ed
string (in the order of execution). The bod
ies of the currently executed "eval" and of
"eval"ed strings that define subroutines are
saved and thus accessible.
/pattern/ Search forwards for pattern (a Perl regex);
final / is optional.
?pattern? Search backwards for pattern; final ? is
optional.
L List all breakpoints and actions.
S [[!]regex]
List subroutine names [not] matching the
regex.
t Toggle trace mode (see also the "AutoTrace"
option).
t expr Trace through execution of "expr". See the
Frame Listing Output Examples entry in the
perldebguts manpage for examples.
b [line] [condition]
Set a breakpoint before the given line. If
line is omitted, set a breakpoint on the line
about to be executed. If a condition is spec
ified, it's evaluated each time the statement
is reached: a breakpoint is taken only if the
condition is true. Breakpoints may only be
set on lines that begin an executable state
ment. Conditions don't use "if":
b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i
b subname [condition]
Set a breakpoint before the first line of the
named subroutine. subname may be a variable
containing a code reference (in this case con_
dition is not supported).
b postpone subname [condition]
Set a breakpoint at first line of subroutine
after it is compiled.
b load filename
Set a breakpoint before the first executed
line of the filename, which should be a full
pathname found amongst the %INC values.
b compile subname
Sets a breakpoint before the first statement
executed after the specified subroutine is
compiled.
d [line] Delete a breakpoint from the specified line.
If line is omitted, deletes the breakpoint
from the line about to be executed.
D Delete all installed breakpoints.
a [line] command
Set an action to be done before the line is
executed. If line is omitted, set an action
on the line about to be executed. The
sequence of steps taken by the debugger is
1. check for a breakpoint at this line
2. print the line if necessary (tracing)
3. do any actions associated with that line
4. prompt user if at a breakpoint or in single-step
5. evaluate line
For example, this will print out $foo every
time line 53 is passed:
a 53 print "DB FOUND $foo\n"
a [line] Delete an action from the specified line. If
line is omitted, delete the action on the line
that is about to be executed.
A Delete all installed actions.
W expr Add a global watch-expression. We hope you
know what one of these is, because they're
supposed to be obvious. WARNING: It is far
too easy to destroy your watch expressions by
accidentally omitting the expr.
W Delete all watch-expressions.
O booloption ...
Set each listed Boolean option to the value
"1".
O anyoption? ...
Print out the value of one or more options.
O option=value ...
Set the value of one or more options. If the
value has internal whitespace, it should be
quoted. For example, you could set "O
pager="less -MQeicsNfr"" to call less with
those specific options. You may use either
single or double quotes, but if you do, you
must escape any embedded instances of same
sort of quote you began with, as well as any
escaping any escapes that immediately precede
that quote but which are not meant to escape
the quote itself. In other words, you follow
single-quoting rules irrespective of the
quote; eg: "O option='this isn\'t bad'" or "O
option="She said, \"Isn't it?\""".
For historical reasons, the "=value" is
optional, but defaults to 1 only where it is
safe to do so--that is, mostly for Boolean
options. It is always better to assign a spe
cific value using "=". The "option" can be
abbreviated, but for clarity probably should
not be. Several options can be set together.
See the section on "Configurable Options" for
a list of these.
< ? List out all pre-prompt Perl command actions.
< [ command ]
Set an action (Perl command) to happen before
every debugger prompt. A multi-line command
may be entered by backslashing the newlines.
WARNING If "command" is missing, all actions
are wiped out!
<< command Add an action (Perl command) to happen before
every debugger prompt. A multi-line command
may be entered by backwhacking the newlines.
> ? List out post-prompt Perl command actions.
> command Set an action (Perl command) to happen after
the prompt when you've just given a command to
return to executing the script. A multi-line
command may be entered by backslashing the
newlines (we bet you couldn't've guessed this
by now). WARNING If "command" is missing, all
actions are wiped out!
>> command Adds an action (Perl command) to happen after
the prompt when you've just given a command to
return to executing the script. A multi-line
command may be entered by backslashing the
newlines.
{ ? List out pre-prompt debugger commands.
{ [ command ]
Set an action (debugger command) to happen
before every debugger prompt. A multi-line
command may be entered in the customary fash
ion. WARNING If "command" is missing, all
actions are wiped out!
Because this command is in some senses new, a
warning is issued if you appear to have acci
dentally entered a block instead. If that's
what you mean to do, write it as with ";{ ...
}" or even "do { ... }".
{{ command Add an action (debugger command) to happen
before every debugger prompt. A multi-line
command may be entered, if you can guess how:
see above.
! number Redo a previous command (defaults to the pre
vious command).
! -number Redo number'th previous command.
! pattern Redo last command that started with pattern.
See "O recallCommand", too.
!! cmd Run cmd in a subprocess (reads from DB::IN,
writes to DB::OUT) See "O shellBang", also.
Note that the user's current shell (well,
their "$ENV{SHELL}" variable) will be used,
which can interfere with proper interpretation
of exit status or signal and coredump informa
tion.
H -number Display last n commands. Only commands longer
than one character are listed. If number is
omitted, list them all.
q or ^D Quit. ("quit" doesn't work for this, unless
you've made an alias) This is the only sup
ported way to exit the debugger, though typing
"exit" twice might work.
Set the "inhibit_exit" option to 0 if you want
to be able to step off the end the script.
You may also need to set $finished to 0 if you
want to step through global destruction.
R Restart the debugger by "exec()"ing a new ses
sion. We try to maintain your history across
this, but internal settings and command-line
options may be lost.
The following setting are currently preserved:
history, breakpoints, actions, debugger
options, and the Perl command-line options -w,
-I, and -e.
|dbcmd Run the debugger command, piping DB::OUT into
your current pager.
||dbcmd Same as "|dbcmd" but DB::OUT is temporarily
"select"ed as well.
= [alias value]
Define a command alias, like
= quit q
or list current aliases.
command Execute command as a Perl statement. A trail
ing semicolon will be supplied. If the Perl
statement would otherwise be confused for a
Perl debugger, use a leading semicolon, too.
m expr List which methods may be called on the result
of the evaluated expression. The expression
may evaluated to a reference to a blessed
object, or to a package name.
man [manpage]
Despite its name, this calls your system's
default documentation viewer on the given
page, or on the viewer itself if manpage is
omitted. If that viewer is man, the current
"Config" information is used to invoke man
using the proper MANPATH or -M manpath option.
Failed lookups of the form "XXX" that match
known manpages of the form perlXXX will be
retried. This lets you type "man debug" or
"man op" from the debugger.
On systems traditionally bereft of a usable
man command, the debugger invokes perldoc.
Occasionally this determination is incorrect
due to recalcitrant vendors or rather more
felicitously, to enterprising users. If you
fall into either category, just manually set
the $DB::doccmd variable to whatever viewer to
view the Perl documentation on your system.
This may be set in an rc file, or through
direct assignment. We're still waiting for a
working example of something along the lines
of:
$DB::doccmd = 'netscape -remote http://something.here/';
Configurable Options
The debugger has numerous options settable using the "O"
command, either interactively or from the environment or
an rc file. (./.perldb or ~/.perldb under Unix.)
""recallCommand"", ""ShellBang""
The characters used to recall command or spawn
shell. By default, both are set to "!", which
is unfortunate.
""pager"" Program to use for output of pager-piped com
mands (those beginning with a "|" character.)
By default, "$ENV{PAGER}" will be used.
Because the debugger uses your current termi
nal characteristics for bold and underlining,
if the chosen pager does not pass escape
sequences through unchanged, the output of
some debugger commands will not be readable
when sent through the pager.
""tkRunning""
Run Tk while prompting (with ReadLine).
""signalLevel"", ""warnLevel"", ""dieLevel""
Level of verbosity. By default, the debugger
leaves your exceptions and warnings alone,
because altering them can break correctly run
ning programs. It will attempt to print a
message when uncaught INT, BUS, or SEGV sig
nals arrive. (But see the mention of signals
in the BUGS manpage below.)
To disable this default safe mode, set these
values to something higher than 0. At a level
of 1, you get backtraces upon receiving any
kind of warning (this is often annoying) or
exception (this is often valuable). Unfortu
nately, the debugger cannot discern fatal
exceptions from non-fatal ones. If "dieLevel"
is even 1, then your non-fatal exceptions are
also traced and unceremoniously altered if
they came from "eval'd" strings or from any
kind of "eval" within modules you're attempt
ing to load. If "dieLevel" is 2, the debugger
doesn't care where they came from: It usurps
your exception handler and prints out a trace,
then modifies all exceptions with its own
embellishments. This may perhaps be useful
for some tracing purposes, but tends to hope
lessly destroy any program that takes its
exception handling seriously.
""AutoTrace""
Trace mode (similar to "t" command, but can be
put into "PERLDB_OPTS").
""LineInfo""
File or pipe to print line number info to. If
it is a pipe (say, "|visual_perl_db"), then a
short message is used. This is the mechanism
used to interact with a slave editor or visual
debugger, such as the special "vi" or "emacs"
hooks, or the "ddd" graphical debugger.
""inhibit_exit""
If 0, allows stepping off the end of the
script.
""PrintRet""
Print return value after "r" command if set
(default).
""ornaments""
Affects screen appearance of the command line
(see the Term::ReadLine manpage). There is
currently no way to disable these, which can
render some output illegible on some displays,
or with some pagers. This is considered a
bug.
""frame"" Affects the printing of messages upon entry
and exit from subroutines. If "frame & 2" is
false, messages are printed on entry only.
(Printing on exit might be useful if inter
spersed with other messages.)
If "frame & 4", arguments to functions are
printed, plus context and caller info. If
"frame & 8", overloaded "stringify" and "tie"d
"FETCH" is enabled on the printed arguments.
If "frame & 16", the return value from the
subroutine is printed.
The length at which the argument list is trun
cated is governed by the next option:
""maxTraceLen""
Length to truncate the argument list when the
"frame" option's bit 4 is set.
The following options affect what happens with "V", "X",
and "x" commands:
""arrayDepth"", ""hashDepth""
Print only first N elements ('' for all).
""compactDump"", ""veryCompact""
Change the style of array and hash output. If
"compactDump", short array may be printed on
one line.
""globPrint""
Whether to print contents of globs.
""DumpDBFiles""
Dump arrays holding debugged files.
""DumpPackages""
Dump symbol tables of packages.
""DumpReused""
Dump contents of "reused" addresses.
""quote"", ""HighBit"", ""undefPrint""
Change the style of string dump. The default
value for "quote" is "auto"; one can enable
double-quotish or single-quotish format by
setting it to """ or "'", respectively. By
default, characters with their high bit set
are printed verbatim.
""UsageOnly""
Rudimentary per-package memory usage dump.
Calculates total size of strings found in
variables in the package. This does not
include lexicals in a module's file scope, or
lost in closures.
After the rc file is read, the debugger reads the
"$ENV{PERLDB_OPTS}" environment variable and parses this
as the remainder of a `O ...' line as one might enter at
the debugger prompt. You may place the initialization
options "TTY", "noTTY", "ReadLine", and "NonStop" there.
If your rc file contains:
parse_options("NonStop=1 LineInfo=db.out AutoTrace");
then your script will run without human intervention,
putting trace information into the file db.out. (If you
interrupt it, you'd better reset "LineInfo" to /dev/tty if
you expect to see anything.)
""TTY"" The TTY to use for debugging I/O.
""noTTY"" If set, the debugger goes into "NonStop" mode
and will not connect to a TTY. If interrupted
(or if control goes to the debugger via
explicit setting of $DB::signal or $DB::single
from the Perl script), it connects to a TTY
specified in the "TTY" option at startup, or
to a tty found at runtime using the
"Term::Rendezvous" module of your choice.
This module should implement a method named
"new" that returns an object with two methods:
"IN" and "OUT". These should return filehan
dles to use for debugging input and output
correspondingly. The "new" method should
inspect an argument containing the value of
"$ENV{PERLDB_NOTTY}" at startup, or
""/tmp/perldbtty$$"" otherwise. This file is
not inspected for proper ownership, so secu
rity hazards are theoretically possible.
""ReadLine""
If false, readline support in the debugger is
disabled in order to debug applications that
themselves use ReadLine.
""NonStop"" If set, the debugger goes into non-interactive
mode until interrupted, or programmatically by
setting $DB::signal or $DB::single.
Here's an example of using the "$ENV{PERLDB_OPTS}" vari
able:
$ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
That will run the script myprogram without human interven
tion, printing out the call tree with entry and exit
points. Note that "NonStop=1 frame=2" is equivalent to "N
f=2", and that originally, options could be uniquely
abbreviated by the first letter (modulo the "Dump*"
options). It is nevertheless recommended that you always
spell them out in full for legibility and future compati
bility.
Other examples include
$ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
which runs script non-interactively, printing info on each
entry into a subroutine and each executed line into the
file named listing. (If you interrupt it, you would bet
ter reset "LineInfo" to something "interactive"!)
Other examples include (using standard shell syntax to
show environment variable settings):
$ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
perl -d myprogram )
which may be useful for debugging a program that uses
"Term::ReadLine" itself. Do not forget to detach your
shell from the TTY in the window that corresponds to
/dev/ttyXX, say, by issuing a command like
$ sleep 1000000
See the Debugger Internals entry in the perldebguts man
page for details.
Debugger input/output
Prompt The debugger prompt is something like
DB<8>
or even
DB<<17>>
where that number is the command number, and which
you'd use to access with the built-in csh-like
history mechanism. For example, "!17" would
repeat command number 17. The depth of the angle
brackets indicates the nesting depth of the debug
ger. You could get more than one set of brackets,
for example, if you'd already at a breakpoint and
then printed the result of a function call that
itself has a breakpoint, or you step into an
expression via "s/n/t expression" command.
Multiline commands
If you want to enter a multi-line command, such as
a subroutine definition with several statements or
a format, escape the newline that would normally
end the debugger command with a backslash. Here's
an example:
DB<1> for (1..4) { \
cont: print "ok\n"; \
cont: }
ok
ok
ok
ok
Note that this business of escaping a newline is
specific to interactive commands typed into the
debugger.
Stack backtrace
Here's an example of what a stack backtrace via
"T" command might look like:
$ = main::infested called from file `Ambulation.pm' line 10
@ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
$ = main::pests('bactrian', 4) called from file `camel_flea' line 4
The left-hand character up there indicates the
context in which the function was called, with "$"
and "@" meaning scalar or list contexts respec
tively, and "." meaning void context (which is
actually a sort of scalar context). The display
above says that you were in the function
"main::infested" when you ran the stack dump, and
that it was called in scalar context from line 10
of the file Ambulation.pm, but without any argu
ments at all, meaning it was called as
"&infested". The next stack frame shows that the
function "Ambulation::legs" was called in list
context from the camel_flea file with four argu
ments. The last stack frame shows that
"main::pests" was called in scalar context, also
from camel_flea, but from line 4.
If you execute the "T" command from inside an
active "use" statement, the backtrace will contain
both a "require" frame and an "eval") frame.
Line Listing Format
This shows the sorts of output the "l" command can
produce:
DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$pack} = ()
103 if(exists $i{$prevpack} || exists $isa{$pack});
104 }
105
106 next
107==> if(exists $isa{$pack});
108
109:a if ($extra-- > 0) {
110: %isa = ($pack,1);
Breakable lines are marked with ":". Lines with
breakpoints are marked by "b" and those with
actions by "a". The line that's about to be exe
cuted is marked by "==>".
Please be aware that code in debugger listings may
not look the same as your original source code.
Line directives and external source filters can
alter the code before Perl sees it, causing code
to move from its original positions or take on
entirely different forms.
Frame listing
When the "frame" option is set, the debugger would
print entered (and optionally exited) subroutines
in different styles. See the perldebguts manpage
for incredibly long examples of these.
Debugging compile-time statements
If you have compile-time executable statements (such as
code within BEGIN and CHECK blocks or "use" statements),
these will not be stopped by debugger, although "require"s
and INIT blocks will, and compile-time statements can be
traced with "AutoTrace" option set in "PERLDB_OPTS").
From your own Perl code, however, you can transfer control
back to the debugger using the following statement, which
is harmless if the debugger is not running:
$DB::single = 1;
If you set "$DB::single" to 2, it's equivalent to having
just typed the "n" command, whereas a value of 1 means the
"s" command. The "$DB::trace" variable should be set to
1 to simulate having typed the "t" command.
Another way to debug compile-time code is to start the
debugger, set a breakpoint on the load of some module:
DB<7> b load f:/perllib/lib/Carp.pm
Will stop on load of `f:/perllib/lib/Carp.pm'.
and then restart the debugger using the "R" command (if
possible). One can use "b compile subname" for the same
purpose.
Debugger Customization
The debugger probably contains enough configuration hooks
that you won't ever have to modify it yourself. You may
change the behaviour of debugger from within the debugger
using its "O" command, from the command line via the
"PERLDB_OPTS" environment variable, and from customization
files.
You can do some customization by setting up a .perldb
file, which contains initialization code. For instance,
you could make aliases like these (the last one is one
people expect to be there):
$DB::alias{'len'} = 's/^len(.*)/p length($1)/';
$DB::alias{'stop'} = 's/^stop (at|in)/b/';
$DB::alias{'ps'} = 's/^ps\b/p scalar /';
$DB::alias{'quit'} = 's/^quit(\s*)/exit/';
You can change options from .perldb by using calls like
this one;
parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
The code is executed in the package "DB". Note that
.perldb is processed before processing "PERLDB_OPTS". If
.perldb defines the subroutine "afterinit", that function
is called after debugger initialization ends. .perldb may
be contained in the current directory, or in the home
directory. Because this file is sourced in by Perl and
may contain arbitrary commands, for security reasons, it
must be owned by the superuser or the current user, and
writable by no one but its owner.
If you want to modify the debugger, copy perl5db.pl from
the Perl library to another name and hack it to your
heart's content. You'll then want to set your "PERL5DB"
environment variable to say something like this:
BEGIN { require "myperl5db.pl" }
As a last resort, you could also use "PERL5DB" to cus
tomize the debugger by directly setting internal variables
or calling debugger functions.
Note that any variables and functions that are not docu
mented in this document (or in the perldebguts manpage)
are considered for internal use only, and as such are sub
ject to change without notice.
Readline Support
As shipped, the only command-line history supplied is a
simplistic one that checks for leading exclamation points.
However, if you install the Term::ReadKey and Term::Read
Line modules from CPAN, you will have full editing capa
bilities much like GNU readline(3) provides. Look for
these in the modules/by-module/Term directory on CPAN.
These do not support normal vi command-line editing, how
ever.
A rudimentary command-line completion is also available.
Unfortunately, the names of lexical variables are not
available for completion.
Editor Support for Debugging
If you have the FSF's version of emacs installed on your
system, it can interact with the Perl debugger to provide
an integrated software development environment reminiscent
of its interactions with C debuggers.
Perl comes with a start file for making emacs act like a
syntax-directed editor that understands (some of) Perl's
syntax. Look in the emacs directory of the Perl source
distribution.
A similar setup by Tom Christiansen for interacting with
any vendor-shipped vi and the X11 window system is also
available. This works similarly to the integrated multi
window support that emacs provides, where the debugger
drives the editor. At the time of this writing, however,
that tool's eventual location in the Perl distribution was
uncertain.
Users of vi should also look into vim and gvim, the mousey
and windy version, for coloring of Perl keywords.
Note that only perl can truly parse Perl, so all such CASE
tools fall somewhat short of the mark, especially if you
don't program your Perl as a C programmer might.
The Perl Profiler
If you wish to supply an alternative debugger for Perl to
run, just invoke your script with a colon and a package
argument given to the -d flag. The most popular alterna
tive debuggers for Perl is the Perl profiler.
Devel::DProf is now included with the standard Perl dis
tribution. To profile your Perl program in the file
mycode.pl, just type:
$ perl -d:DProf mycode.pl
When the script terminates the profiler will dump the pro
file information to a file called tmon.out. A tool like
dprofpp, also supplied with the standard Perl distribu
tion, can be used to interpret the information in that
profile.
Debugging regular expressions
"use re 'debug'" enables you to see the gory details of
how the Perl regular expression engine works. In order to
understand this typically voluminous output, one must not
only have some idea about about how regular expression
matching works in general, but also know how Perl's regu
lar expressions are internally compiled into an automaton.
These matters are explored in some detail in the Debugging
regular expressions entry in the perldebguts manpage.
Debugging memory usage
Perl contains internal support for reporting its own mem
ory usage, but this is a fairly advanced concept that
requires some understanding of how memory allocation
works. See the Debugging Perl memory usage entry in the
perldebguts manpage for the details.
SEE ALSO
You did try the -w switch, didn't you?
the perldebguts manpage, the re manpage, the DB manpage,
the Devel::Dprof manpage, the dprofpp manpage, the Dump
value manpage, and the perlrun manpage.
BUGS
You cannot get stack frame information or in any fashion
debug functions that were not compiled by Perl, such as
those from C or C++ extensions.
If you alter your @_ arguments in a subroutine (such as
with "shift" or "pop", the stack backtrace will not show
the original values.
The debugger does not currently work in conjunction with
the -W command-line switch, because it itself is not free
of warnings.
If you're in a slow syscall (like "wait"ing, "accept"ing,
or "read"ing from your keyboard or a socket) and haven't
set up your own "$SIG{INT}" handler, then you won't be
able to CTRL-C your way back to the debugger, because the
debugger's own "$SIG{INT}" handler doesn't understand that
it needs to raise an exception to longjmp(3) out of slow
syscalls.
2001-04-07 perl v5.6.1 PERLDEBUG(1)
